3. Frequency analysis

Frequency analysis introduces the notion of power spectral density (PSD). Before developing this notion of PSD, a few precautions are necessary to ensure the representativeness of the developments. These precautions concern the nature of the stresses to be studied. It is assumed that the trajectory relative to the stress process X(t) satisfies the following hypotheses:

• the process X(t) is continuous over the chosen observation time, equal to a unit of time referred to as the reference period;

• the first $$